The study of half a million Danish women who received the HPV vaccine, which protects against cervical cancer and other health problems, found no link between the shots and the formation of blood clots known as venous thromboembolisms. In the United States it's called the Gardasil vaccine. Concerns about such a link had been raised in previous research.

"Our results found no evidence to support that exposure to the HPV vaccine is associated with an increased risk of [venous thromboembolisms] in the period following vaccination," said Nikolai Madrid Scheller, a researcher at Statens Serum Institute in Copenhagen.

HPV vaccines are recommended for 11- and 12-year-olds because they work best before the start of sexual activity.

The researchers conducted the new study, published July 9 in the Journal of the American Medical Association, after earlier research found a risk of blood clots in the weeks after vaccination.

For the new study, the researchers looked at data on 1.6 million Danish females ages 10 through 44. There were 4,375 cases of blood clots among more than 500,000 teens and women who got the vaccine. Of these, one-fifth were vaccinated during the study period, from 2006 to 2013.

The researchers analyzed the cases in multiple ways. They first looked at all patients with blood clots -- those who did and didn't get the vaccine.

They also looked only at patients who had received the vaccine. They analyzed different age groups and adjusted for oral contraceptive use, which is known to increase blood clot risk. They zeroed in on the 42-day window after vaccination, considered the most high-risk period for clots, and also looked at a control period, before and after the vulnerable period.

Despite the different analyses, no link was found between the vaccine and a higher risk of getting a blood clot in the vulnerable time period.